1. Field of the Invention:
The present invention relates to a UV-crosslinkable organopolysiloxane composition and to the use thereof as a negative resist especially adapted for microlithography applications.
2. Description of the Prior Art:
Significant progress has been made in recent years, both in the size reduction and in the cost reduction of electronic components, by virtue of the advances made in the technology of production of microelectronic circuits, for example, in the field of microlithography.
In general, the microlithography process includes:
(a) the application of "a resist", a film of radiation-sensitive polymeric material to one of the face surfaces of the substrate,
(b) the exposure of certain areas of the film to ultraviolet light, an electron beam, X-rays, and the like, for example, and
(c) development of the film with a solvent to remove the soluble portions thereof.
Insofar as the polymeric materials, the "resists", are concerned, these are commonly divided into two categories, according to their behavior when subjected to irradiation.
Some resins are characterized by an increased solubility after irradiation and produce a hollow image: these resins are referred to as positive. The others become insoluble in a solvent for post-irradiation development and they produce a relief image; these resins are referred to as negative.
In each case, the resist portion which remains on the substrate is employed as a protective coating to permit selective etching or any other treatment of the uncoated areas of the substrate.
Substrate etching may be performed by conventional chemical attack or by a plasma. Plasma etching generally makes it possible to obtain finer resolution than that obtained by chemical attack, and, in addition, makes it possible to eliminate contamination and handling problems which are inherent in chemical agents. However, many resins cannot withstand plasma discharge and are themselves eroded along with the substrate, and this unavoidably gives rise to a loss in image resolution.
In parallel with these approaches, it will be appreciated that the growing complexity and miniaturization of integrated circuits expose the limitations of the technologies which rely on only one resist layer or level. Thus, in various earlier investigations reported, for example, in Solid State Technology, pages 130-135 (Aug., 1985), it has been proposed to solve, at least partially, the problems presented by the topography and the reflection of a substrate such as a silicon wafer, by using a plurality of resist layers or levels.
Thus, it has been proposed to deposit a first, relatively thick layer of a polymeric material, referred to as a flattening layer, and to coat this first thickness with a thin radiation-sensitive, plasma-resistant layer of a silicon-containing polymer.
There exists, therefore, serious need in this art for polymeric materials having enhanced radiation sensitivity for producing repeat patterns with higher resolution, in response to the demand for increasingly sophisticated circuitry.
Understandably, this research is carried out in parallel with the emergence of the newer technologies.
As will be seen later, the present invention relates more particularly to the field of negative resists, that is to say, those polymeric compositions or materials which are rendered or become insoluble in a development solvent after exposure to light, especially to ultraviolet (UV) radiation.
There already exist in this art known systems in which an initiator of chemical reactions, reactions which involve either the principal chain or the side groups of a polymer and which produce a profound change in the solubility and/or in the plasma-etching resistance of this polymer, is activated under the influence of UV-radiation.
Thus, U.S. Pat. No. 3,984,253 describes the sensitization of a polyphthalaldehyde to UV-radiation, to an electron beam, or to X-rays, by the introduction of acid-generating compounds such as diazonium salts in order to produce a positive image.
There is another known system (cf. U.S. Pat. No. 4,491,628) in which an acid is generated by irradiation, and the acid generated in such manner is used to cleave labile pendent acid groups on the side chains of a polymer. The irradiated portions differ from the unexposed portions in their polarity and their solubility; they can, therefore, be selectively removed using alkaline development treatments or polar solvents. The unexposed portions may be selectively removed using a nonpolar solvent. However, most of the compositions which can be used for this purpose form, after development, solid layers which do not withstand plasma. Furthermore, the choice of the solvents employed for development is very critical.
It is also known to this art that the polysiloxanes are among the foremost polymers envisaged for the preparation of two-layer resists. However, these dimethylsiloxane, methylphenylsiloxane and/or methylvinylsiloxane copolymers are sensitive to near UV-radiation only insofar as care has been taken to incorporate sensitive groups into their structure, for example, acrylic or methacrylic groups. Furthermore, incorporation of such functional groups in the polymer structure is difficult to carry out.
It has also been proposed (cf. JA 60/057,833) to use, as a resist capable of withstanding an oxygen plasma, polysiloxanes having what is known as a "ladder" structure of the formula: ##STR1## in which l, m and n are positive integers which may be zero, provided, however, that l and m are not zero simultaneously.
Notwithstanding the fact that these polymers are relatively difficult to synthesize, they also constitute part of a system in which a direct photosensitization, involving substantially pure photochemistry, occurs, that is to say, each photon activates a pendent chemical moiety on the principal chain of the polymer, and this implies that the degree of crosslinking or chain scission which is detected, depending on each particular case, is directly proportional to the amount of light energy which is applied.